A way to bet on smaller Chinese companies is getting punished Tuesday, after being this year’s top-performing U.S.-listed ETF at the halfway mark.

The Market Vectors ChinaAMC SME-ChiNext ETF
CNXT, -0.67%
was down about 20% at midday, putting it on pace for its biggest one-day percentage drop since its launch a year ago. The fund’s largest dive has been a 14.8% dive on June 26.

The Chinese equities ETF’s price has been cut in half in about 3½ weeks, as shown in the chart below. That’s come amid a selling frenzy in China that’s knocked $2.8 trillion in value from that country’s stock markets, prompting government rescue efforts that have been met with skepticism.

The Market Vectors ChinaAMC SME-ChiNext ETF had been up 64.7% for the year as of June 30, outperforming all other nonleveraged U.S. exchange-traded products. But the fund’s 2015-to-date gain now has been trimmed to about 15% as of midday Tuesday, making it far from being the year’s big winner.

The ETF provides exposure to 100 smaller- and medium-size Chinese companies, tilting toward tech, consumer-related firms and startups, according to ETF.com analysts. It has attracted $48 million in investor money.

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